Methanol is widely used in different applications such as: the synthesis of formaldehyde, which is then involved in the manufacture of plastic materials, paints, and textiles, for instance; the production of dimethylether, which may be used in aerosols or as an alternative fuel for diesel engines; the transesterification of triglycerides to produce biodiesel; or as a solvent or a fuel for engines.
Methanol is commercially produced from synthesis gas (syngas), i.e., a mixture of carbon oxide (i.e., carbon monoxide (CO) and/or carbon dioxide (CO2)) and hydrogen (H2) that can be produced from a variety of carbonated sources. CO and CO2 react with H2 according to the following equations:CO+2H2═CH3OH  (1)CO2+3H2=CH3OH+H2O  (2)CO+H2O=CO2+H2  (3)wherein the third one corresponds to the water-gas shift (WGS) reaction.
A widely used catalyst is Cu/ZnO/Al2O3 (herein after “CuZnAl”), described for instance in GB1159035.
Usually, a syngas feed stream containing about 3 vol % CO2 is used in the methanol synthesis process. This amount of 3 vol % is an optimal value since it is has been demonstrated that increasing the content of CO2 in the syngas feed is detrimental to methanol productivity with CuZnAl catalyst due to the large amount of co-produced water, which strongly inhibits the catalyst activity and results in the loss of catalyst stability (Sahibzada, M. et al.; J. Catal. 1998, 174, 111-118; Martin, O. and Perez-Ramirez, J.; Catal. Sci. Technol. 2013, 3, 3343-3352). Water is produced directly in the hydrogenation of CO2 to methanol, and also in the reverse water-gas shift (RWGS) reaction which competes with the hydrogenation reaction.
It would be desirable to use larger amounts of CO2 in the production of methanol because CO2 is a green-house gas intimately related to industrial activity and modern society and therefore such a use could help in reducing the CO2 footprint of industries.
Most of the current research in methanol synthesis from CO2 has been focusing on the optimization of the commercially-available CuZnAl catalyst to prevent its deactivation in the presence of water or to inhibit the RWGS reaction, as described for example in “Zinc-rich copper catalysts promoted by gold for methanol synthesis” by Martin, O. et al.; ACS Catal. 2015, 5, 5607-5616.
In spite of the improvements, these issues have not been overcome entirely. Thus, novel catalyst formulations have been investigated, such as: Cu—ZnO—Ga2O3/SiO2 in Toyir, J. et al.; Appl. Catal., B 2001, 29, 207-215; or Pd—ZnO/CNT in Liang, X. L. et al.; Appl. Catal., B 2009, 88, 315-322; or Cu/TaC in Dubois, J. L. et al.; Chem. Lett. 1992, 21, 5-8; or LaCr0.5Cu0.5O3 in Jia, L. et al.; Catal. Comm. 2009, 10, 2000-2003.
Of these, only Cu—ZnO—Ga2O3/SiO2 displayed both high activity and selectivity (99.5%). However, also optimized catalysts with low Cu content have been shown to suffer from H2O inhibition, limiting their exploitation only at low conversion levels (Martin et al.; ACS Catal. 2015, 5, 5607-5616). Moreover, little or no data exist to evaluate the long-term stability of such catalysts in the CO2 hydrogenation reaction to methanol.
Indium oxide (In2O3) has recently been identified as a potential good catalyst for CO2 hydrogenation into methanol based on density-functional-theory calculations in “Active Oxygen Vacancy Site for Methanol Synthesis from CO2 Hydrogenation on In2O3(110): a DFT study” Ye, J. et al.; ACS Catal. 2013, 3, 1296-1306. This study indicates that the oxygen-defective In2O3(110) surface can activate CO2 and hydrogenate it via HCOO and H3CO species to methanol. An experimental study over commercially available In2O3 demonstrated reasonable CO2 conversion for this catalyst but only low selectivity in “Hydrogenation of CO2 to methanol over In2O3catalyst” Sun, K. et al., J. CO2 Util. 2015, 12, 1-6.
GB1496379 discloses the dehydrocyclodimerisation of C4 feedstock using a catalyst containing germanium, indium or tin metal or a compound thereof deposited on a support; it exemplifies passing isobutene over GeO2, In2O3 and SnO2 deposited on eta alumina.
GB706599 discloses the reaction of CO with H2 using as catalysts oxides of group IVa (Ti, Zr, Hf) to which may be added activating oxides of trivalent metals; a combination of zirconium oxide with indium oxide and potassium oxide is disclosed as especially suitable. The catalyst is preferably prepared by precipitation with alkalies from an aqueous solution.
Only carbon monoxide is reacted with hydrogen. The objective stated in GB706599 is to increase the yield of isobutyl alcohol.
U.S. Pat. No. 5,506,273 relates to the hydrogenation of CO and CO2 with a catalyst essentially consisting of gold and at least one metal oxide, the gold being in the form of particles deposited on the oxide, both possibly being supported; the exemplified metal oxides are ZnO, Fe2O3, TiO2, ZrO2, La2O3, ZnFe2O4 and CeO2.
There is still a need for a process of methanol synthesis and a catalyst showing high stability, high selectivity, high activity, and low inhibition by water.